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Variations in climate produced observable differences in the thickness of sediments, and, like the patterns of variation in tree rings, this allows matches to be made between deposits in separate lake beds.
Microscopic wind-blown pollen grains survive well in many soil conditions, and pollen that has accumulated in deep deposits - such as peat-bogs - can provide a long-term record of changes in vegetation; suitable samples may be collected from soils exposed by excavation, or from cores extracted from bogs.
It is increasingly difficult for prehistorians working in the twenty-first century to conceptualise the problems experienced by their predecessors, and approaches to interpretation before the 1960s are consistently criticised.
Culture history and diffusionism may - with hindsight - seem excessively preoccupied with classification and social evolution, and to have applied unsophisticated historical interpretations instead of asking fundamental questions about human behaviour.
Like thermoluminescence, ESR is a 'trapped charge' dating method, but it is applied to different kinds of samples, and the method of measurement is also different.
The transformation of archaeological dating that began around 1950 continues, but archaeologists may overlook the revolution in scientific dating that had already taken place in geology during the first half of the twentieth century; from this wider perspective, the emergence of radiocarbon dating may seem slightly less dramatic Accurate knowledge of the age of the Earth was of little direct help to archaeologists, but it emphasised the potential of scientific dating techniques.
The first half of the twentieth century witnessed similar progress that began with the dating of recent geological periods in which early hominids lived, and ended with the introduction of radiocarbon dating.
Cores extracted from ocean floor deposits reveal variations in oxygen isotopes in the shells and skeletal material of dead marine creatures, which reflect fluctuations in global temperature and the volume of the ocean.
Sections cut through lake beds in glacial regions reveal a regular annual pattern of coarse and fine layers, known as varves.